drm/i915: Split out !RCS legacy context switching

Having the !RCS legacy context switch threaded through the RCS switching
code makes it much harder to follow and understand. In the next patch, I
want to fix a bug handling the incomplete switch, this is made much
simpler if we segregate the two paths now.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460565315-7748-14-git-send-email-chris@chris-wilson.co.uk